The present invention is a magnetic displacement sensor having a magnetic circuit reluctance that is constant with displacement. More specifically, the present invention is a magnetic displacement sensor having improved flux shaping pole pieces for improved performance.
Typically, magnetic displacement sensors include a flux generator that provides a constant source of magnetic flux and a pickup device that measures flux. Typically, the flux generator is mounted to one element and the pickup device is mounted to another element so that the magnetic flux density sensed by the pickup device is based on the displacement between the elements. Magnetic displacement sensors typically measure linear or rotational displacement and provide an output proportional to absolute linear or rotary position displacement of the elements. Magnetic displacement sensors may employ either electromagnets or permanent magnets as a source of magnetic flux. A pickup device (e.g., a magnetoresistor, a magnetodiode, or a Hall effect sensor) intersects the magnetic flux and detects changes in the magnetic field produced by the magnets.
Magnetic displacement sensors are commonly used in cooperation with microprocessors in remote control systems with field devices. For example, magnetic displacement sensors can be used to monitor valve position. Examples of prior art magnetic displacement sensors are found in Prinz et al. U.S. Pat. No. 4,532,810, Wolf et al. U.S. Pat. No. 5,497,081, and Riggs et al. U.S. Pat. No. 5,359,288.
The useful range of prior art magnetic displacement sensors is limited by the magnets' fringing flux. When two permanent magnets are adjacently aligned, the mechanical characteristic of each magnet and its proximity of another magnet pole face dictate the distribution of magnetic flux from pole face to opposite pole face. At the magnetic poles, fringing flux appears between the magnets. Fringing flux variations are non-linear and, therefore, the magnetic field detected by the magnetic displacement sensor varies non-linearly with displacement. This non-linearity results in inaccurate and erroneous sensor readings, limiting the useful range and effectiveness of magnetic displacement sensors.